During medical procedures such as administration of an epidural anesthetic, it is necessary to position the tip of a needle at a relatively precise position inside the patient's body. During procedures involving epidural anesthesia, the needle tip is located in the potential epidural space. If the needle tip has been advanced too far into the body, it projects through the dura mater into the subarachnoid space, creating an opening out of which cerebral spinal fluid may leak, the leakage being associated with post-dural puncture headache. A ground glass syringe is often utilized in this procedure to determine the location of the needle tip through the use of pressure variations within various body spaces. Because a syringe is normally considered a disposable item, it is desirable to reduce its cost to the hospital and patient below that of the relatively expensive ground glass syringes. Glass syringes have a propensity to break if dropped, and have been know to freeze in position in the barrel, necessitating immediate replacement with a new syringe before continuation of the procedure.
A need exists for a syringe which will ease the demands placed on the anesthesiologist's dexterity in performing administration of an epidural anesthetic without resort to unfamiliar devices which require special handling or are difficult to use, and while providing the sensitivity of feel that glass syringes are known for. The precision fit between a glass plunger and a glass barrel confers a sensitive feel to the user when air or liquid flows into or exits the syringe. The frictional resistance between the glass plunger and glass barrel is very low and thus a change in pressure inside the syringe barrel is communicated to the user by a corresponding movement of the plunger. Conversely, the frictional resistance between an elastomeric plunger tip and a plastic syringe barrel is very high due to drag created by rubbing of the outer diameter of the plunger tip against the inner diameter of the plastic barrel during movement.
There is a need in the art for, and thus it is one object of the invention to provide, a plastic syringe which has the sensitive feel and low resistance of a glass syringe.
It is also an object of the invention to provide a simple, reliable, inexpensive modification of a conventional syringe, in particular for use with syringes of the so-called "loss of resistance" type which are specifically designed for low barrel friction to facilitate kinesthetic sensing of changes in plunger resistance to injection as the needle passes through various anatomical structures.
U.S. Pat. No. 4,266,557, to Merry, describes a low friction syringe in which the forward and rearward ends of the gasket portion of the plunger includes first and second annular flanges, respectively. Each flange narrows into a hinge region and then widens into an enlarged lip. The enlarged lip flexes at the hinge region and relative to the remainder of the flange. The low resistance syringe of the invention is an improvement over the low resistance syringe described in U.S. Pat. No. 4,266,557 in that the syringe of the invention contains an annular projection extending from each flange and of reduced thickness relative to the thickness of the flange, instead of the narrow hinge region and wider enlarged outer lip of the '557 patent. The resultant reduced drag of the annular projections on the inner surface of the barrel in the syringe of the invention is an improvement with respect to sensitivity and feel, and thus also with respect to detecting loss of resistance in delicate procedures such as location of the epidural space.